U.S. Pat. No. 4,168,570 discloses a shaving appliance, which comprises a housing, a stationary cutting element, and a rotatable cutting element, the stationary cutting element taking the form of a cap in which hair-entry apertures are formed, which cap is positioned inside the housing by means of a rim and is depressable in an inward direction relative to the housing against spring force. During a shaving action, the cap is moved across a portion of skin, catching hairs to be cut off in the process, while preventing direct contact between the skin and the rotatable cutting element.
In particular, the cap is disposed in an opening of a plate-like cap holder which forms part of the housing. The rim of the cap is provided with a flange, which engages a receding rim of the cap holder, wherein the cap is positioned such that it projects from the cap holder as far as possible. The cap is biased in an outward direction with respect to the cap holder by means of a resilient element. Also, the rotatable cutting element exerts an outwardly directed resilient force on the cap, because a drive spindle for the rotatable cutting element is axially supported by a resilient element.
The shaving appliance known from U.S. Pat. No. 4,168,570 comprises three caps. On the basis of the resilient bias of the caps as mentioned, it is possible to maintain contact between all three caps and the portion of skin to be subjected to a shaving action, for various shapes of the portion of skin.
A problem associated with the known shaving appliance is that the rotating cutting element initiates vibrations of the cap. The amplitude of the vibrations of the cap is dependent on the resonant frequency of the cap. When the frequency of the vibrations is close to the resonant frequency of the cap, the cap will start to resonate and the amplitude of the cap vibrations will strongly increase, resulting in a high noise production, or even in permanent damage of the cap and/or other elements of the shaving appliance. In general, the risk of resonating of the cap is high in the case of a relatively thin cap, i.e. a cap having a relatively small material thickness, as the resonant frequency of such a cap consequently appears to be within the range of operating frequencies of the rotatable cutting element. In general, caps are often made as thin as possible in order to ensure a close shave. Therefore, the high noise production is a widespread problem in the field of shaving appliances and similar hair cutting appliances which are equipped with at least one cap as described in the foregoing.